NEW YORK (Reuters) - Fitch Ratings said on Wednesday it may cut Merrill Lynch & Co Inc's (MER.N: Quote, Profile, Research) debt rating due to expected ongoing write-downs and diminished prospects for earnings.

Fitch placed Merrill's long-term issuer default rating of "A-plus," the fifth-highest investment grade level, on review for downgrade, but affirmed the No. 3 Wall Street investment bank's short-term ratings.

"The rating action results in part from diminished expectations that Merrill's fixed income, currency and commodities operation will attain a sustainable level of core profitability in the near term," Fitch said in a statement.

Fitch said the rating action on Merrill follows its review of the investment banking industry. It also affirmed ratings for Lehman Brothers LEH.N, Goldman Sachs (GS.N: Quote, Profile, Research), and Morgan Stanley (MS.N: Quote, Profile, Research).

The rating outlook on Lehman and Morgan Stanley is negative, while Goldman's is stable. The outlook indicates the likely direction of the rating in one to two years.

(Reporting by Anastasija Johnson, Editing by Chizu Nomiyama)